Neurons in the central and peripheral nervous systems degenerate as a normal function of human development and aging. Pathological neuron degeneration, however, is a serious condition seen in several neurological disorders. Neuronal degeneration can be specific or diffuse, and can lead to sensory, motor and cognitive impairments. Neurodegenerative disorders encompass a range of seriously debilitating conditions including Parkinson's disease (PD), amyotrophic lateral sclerosis (ALS, “Lou Gehrig's disease”), multiple sclerosis, Huntington's disease (HD), Alzheimer's disease, pantothenate kinase associated neurodegeneration (PKAN, formerly Hallervorden-Spatz syndrome), multiple system atrophy, diabetic retinopathy, multi-infarct dementia, macular degeneration, and the like. These conditions are characterized by a gradual but relentless worsening of the patient's condition over time. These disorders affect a large population of humans, especially older adults. Nevertheless, the understanding of these disorders is extremely limited and incomplete.
Many advances have been made in gaining a better understanding of PD, Alzheimer's disease and HD. The primary cause of cognitive dysfunction for all three disorders has been directly linked to neuron degeneration, usually in specific areas of the brain. PD is linked to degeneration of neurons in the substantia nigra, while Alzheimer's disease is in some part due to loss of pyramidal neurons in the limbic cortex (Braak, E. & Braak, H., 1999, In: V. E. Koliatsos & R. R. Ratan (eds.), Cell Death and Diseases of the Nervous System, Totowa, N.J.: Humana Press, pp. 497-508). HD's cognitive deficits are produced by degeneration of cells in the caudate nucleus of the striatum. However, although the symptoms and progression of these diseases are well characterized, the causes and triggers at onset are not well understood.
Thus, several strategies are being pursued to develop new therapies for neurodegenerative disorders, including PD. For PD, the techniques range from the use of dopaminotrophic factors (Takayama et al., Nature Med. 1:53-58, 1995) and viral vectors (Choi-Lundberg et al., Science 275:838-841, 1997) to the transplantation of primary xenogeneic tissue (Deacon et al., Nature Med. 3:350-353, 1997). Transplantation of dopaminergic neurons is a clinically promising experimental treatment in late stage PD. More than 200 patients have been transplanted worldwide (Olanow et al., Trends Neurosci. 19:102-109, 1996), and clinical improvement has been confirmed (Olanow et al., supra, and Wenning et al., Ann. Neurol. 42:95-107, 1997) and was correlated to good graft survival and innervation of the host striatum (Kordower et al., N. Engl. J. Med. 332:1118-1124, 1995). However, fetal nigral transplantation therapy generally requires human fetal tissue from at least 3-5 embryos to obtain a clinically reliable improvement in the patient. A different source of these neurons is clearly needed.
Dopaminergic neurons have been generated from murine CNS precursor cells (PCT Application No. PCT/US99/16825; Studer et al., Nature Neurosci. 1:290-295, 1998). These precursor-derived neurons are functional in vitro and in vivo and restore behavioral deficits in a rat model of PD. Even though the primary mesencephalic CNS stem cell culture can provide differentiated dopaminergic neurons suitable for use in cell therapy, the cell number provided by this method is limited. The percentage of differentiated dopaminergic neurons obtained from expanded mesencephalic precursors decreases as the cells are expanded more than about 10-100 fold. Mesencephalic precursors can generate only about 10% to 15% dopaminergic neurons (out of total cell number) after 10-100 fold expansion, and when the precursors are expanded 1000 fold, that number drops further, to only about 1%. Thus, a need clearly remains for alternate sources of these cells. In addition, there is a need for reliable methods for generating larger numbers of primate neurons.